This invention relates to detecting the output signal from a dc superconducting quantum interference device (dc-SQUID) and, more particularly, to the application of digital signal processing to detecting the output signal from a dc-SQUID.
A dc-SQUID is a sensitive device that is used to detect and quantify very weak magnetic signals, e.g., human or animal brain signals for magnetoencephalography studies. It will be appreciated that the magnetic field strength of a brain is on the order of 10.sup.-13 Teslas (T), while background magnetic fields from the earth are on the order of 10.sup.-5 T. Thus, the general background noise level is eight orders of magnitude greater than the signal sought to be detected.
In one approach to reducing the noise to signal conditions, dc-SQUID are used in magnetically shielded rooms. Adequately shielded rooms are complex to design and maintain and are not suitable for routine use of a dc-SQUID. Other techniques have included the use of superconducting coils arranged to cancel the input of extraneous magnetic noise and the use of superconducting surfaces to deflect the fields of distant magnetic sources away from the signal detection coils of the dc-SQUID. These techniques are relatively complex and the efficacy is subject to mechanical features and variations thereof.
Yet another problem with a dc-SQUID is the need to provide for signal tracking over a wide dynamic range. With conventional analog circuits, the range of measurable signals can be increased by selecting or adjusting the gain of that circuit. However, decreasing the gain of a circuit to allow measurement of larger signals will decrease its sensitivity to weak signals. In the gain selection method, the percentage error and the noise floor are some fraction of the peak signal allowed by any given range. That is, the dynamic range is usually some constant for all ranges. Further, the dynamic range of analog measuring systems is limited to the ratio of the peak signal allowed by the electronics to its noise floor.
Still another problem with conventional dc-SQUID signal circuits is maintaining linearity over a wide dynamic range. The operation of a dc-SQUID is well known, that is, the dc-SQUID has a bias current to bias the dc-SQUID in a transition region between a superconducting and normal conducting state, wherein the dc-SQUID outputs a periodic junction voltage as a function of applied magnetic flux, .PHI., where the period is .PHI..sub.o (defined as one flux quantum or /2e). A conventional analog circuit flux locked loop (FLL) holds flux within the SQUID to a constant value, i.e., the loop feedback current is used to cancel changes in flux induced by the input current. When the feedback current exceeds the maximum than can be supplied by the feedback circuit, flux lock is lost; the flux lock loop must be reset to a new lock point. A flux lock error can cause the lock point magnitude to be reset to the nearest multiple of .PHI..sub.o /2. If the lock is moved to an adjacent flux cycle, there will be a gap between the flux lock points with a concomitant accumulation of errors in the output signal.
In accordance with my invention a digital flux-tracking loop (dFTL) is provided with a novel modulation-demodulation technique for extending the usable measurement range of the dc-SQUID by many orders of magnitude without compromising accuracy or resolution. As hereinafter used, the term "SQUID" means dc-SQUID.
Accordingly, one object of the present invention is to achieve a large dynamic range for the SQUID through continuous tracking of the flux quanta over the entire magnetic flux range of SQUID operation.
Another object of the present invention is to provide linearity for the dFTL through accurate determination of .PHI..sub.o in a modulation feedback loop.
Still another object of the present invention is to provide independent orthogonal feedback for maintaining flux tracking and modulation lock.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.